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United States Patent |
5,548,183
|
Miyoshi
,   et al.
|
August 20, 1996
|
Magnetic field immersion type electron gun
Abstract
In a magnetic field immersion type electron gun for controlling an electron
beam emitted by an electron gun (51) with the use of an electric lens (56)
and a magnetic field lens formed by permanent magnets (57, 58) of a
coaxial ion pump (53), the ion pump magnets are a pair of cylindrical
permanent magnets (57, 58) disposed coaxially with an optical axis (52) of
the electron gun (51) in such a way as to sandwich a cylindrical ion pump
anode (61) of the coaxial ion pump; the two permanent magnets are
magnetized in a mutually opposing direction; a hollow cylindrical yoke
(60) is disposed also coaxially with the optical axis (52) in such a way
as to enclose the two permanent magnets (57, 58) within a hollow portion
thereof; and the yoke (60) is formed with an annular yoke gap (63) in a
radially inner circumferential surface of the yoke (60) to leak out a
magnetic flux flowing through the yoke toward the optical axis. In the
above-mentioned construction, the magnetic field lens can be formed
efficiently with the use of the magnetic field generated by the permanent
magnets for constituting the coaxial ion pump, and further the formed
magnetic field lens can be superimposed upon the electron gun. Therefore,
an electric field immersion type electron gun of high performance can be
obtained, and further the electron gun chamber can be efficiently
evacuated in the vicinity of the cathode tip of the electron gun.
Inventors:
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Miyoshi; Motosuke (Tokyo, JP);
Okumura; Katsuya (Poughkeepsie, NY);
Yamazaki; Yuichiro (Tokyo, JP)
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Assignee:
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Kabushiki Kaisha Toshiba (Kawasaki, JP)
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Appl. No.:
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364747 |
Filed:
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December 27, 1994 |
Foreign Application Priority Data
Current U.S. Class: |
313/153; 313/7; 313/310; 313/442; 313/443; 315/85 |
Intern'l Class: |
H01J 003/20 |
Field of Search: |
313/7,442,443,336,153,310
315/85
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References Cited
U.S. Patent Documents
4397611 | Aug., 1983 | Wiesner et al.
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4890029 | Dec., 1989 | Miyoshi et al.
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4945247 | Jul., 1990 | Kawasaki et al. | 313/443.
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5021702 | Jun., 1991 | Miyoshi et al.
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5109179 | Apr., 1992 | Faillon et al. | 313/153.
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Other References
Yamazaki et al; "Development of the field emission electron gun integrated
in the sputter ion pump"; 82576 Journal of Vacuum Science & Technology B 9
(1991) Nov./Dec., No. 6.
Troyon; "High current efficiency field emission gun system incorporating a
preaccelerator magnetic lens"; Optik 57, No. 3 (1980), pp. 401-410.
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Primary Examiner: Patel; Nimeshkumar D.
Attorney, Agent or Firm: Oblon, Spivak, McClelland, Maier & Neustadt, P.C.
Claims
What is claimed is:
1. A magnetic field immersion type electron gun for controlling an electron
beam by an electric field lens and a magnetic field lens in combination,
comprising:
an electron gun body for emitting the electron beam;
an electric field lens system disposed under said electron gun body, for
forming the electric field lens to control the electron beam emitted by
said electron gun body; and
a coaxial ion pump including:
a cylindrical anode and at least one cathode;
a pair of cylindrical permanent magnets disposed coaxially with an optical
axis of said electron gun body in such a way as to sandwich said
cylindrical anode and cathode therebetween and magnetized in a mutually
opposing direction; and
a hollow cylindrical yoke disposed also coaxially with the optical axis
thereof in such a way as to enclose said two permanent magnets within a
hollow portion thereof, said hollow cylindrical yoke being formed with an
annular yoke gap in a radially inner circumferential surface thereof to
leak out a magnetic flux flowing through said hollow cylindrical yoke
toward the optical axis thereof so that a magnetic field lens can be
formed and further superimposed upon the electric field lens formed by
said electric field lens system.
2. The magnetic field immersion type electron gun of claim 1, wherein a
height position of the yoke gap along the optical axis roughly matches a
vertical position of an electron gun cathode attached to a tip portion of
said electron gun body.
3. The magnetic field immersion type electron gun of claim 1, wherein a
pair of said permanent magnets are disposed in such a way as to sandwich
the ion pump anode and cathode in a direction perpendicular to the optical
axis.
4. The magnetic field immersion type electron gun of claim 1, wherein a
pair of said permanent magnets are disposed in such a way as to sandwich
the ion pump anode and cathode in a direction parallel to the optical
axis.
5. The magnetic field immersion type electron gun of claim 1, wherein the
yoke gap is formed at a lower end portion of the radially inner
circumferential surface of said hollow cylindrical yoke so as to face an
electron gun cathode.
6. The magnetic field immersion type electron gun of claim 1, wherein a
pair of said permanent magnets are disposed within a vacuum vessel of said
coaxial ion pump.
7. The magnetic field immersion type electron gun of claim 1, wherein said
permanent magnets are formed of samarium cobalt.
8. A magnetic field immersion type electron gun, comprising:
an electron gun body;
an electric field lens for controlling an electron beam emitted from said
electron gun body; and
a magnetic field lens formed by ion pump magnets of a coaxial ion pump, for
controlling the electron beam emitted by said electron gun body,
wherein:
the ion pump magnets include:
a pair of cylindrical permanent magnets disposed coaxially with an optical
axis of said electron gun body in such a way as to sandwich a cylindrical
ion pump anode of the coaxial ion pump, the two permanent magnets being
magnetized in a mutually opposing direction; and
a hollow cylindrical yoke disposed coaxially with the optical axis in such
a way as to enclose said two permanent magnets within a hollow portion
thereof, said yoke being formed with an annular yoke gap for leaking out a
magnetic flux flowing through said yoke toward the optical axis in a
radially inner circumferential surface thereof facing the optical axis.
Description
BACKGROUND OF THE INVENTION
The present invention relates to a magnetic field immersion type electron
gun for controlling electrons emitted from an electron gun by an electric
field lens and further by a magnetic field lens formed by ion pump magnets
of a coaxial ion pump.
In the electron gun used for an electron microscope, a large intensity is
required for an electron beam emitted from the electron gun, and further
the electron beam is required to be controlled by a lens of a small
spherical aberration. Although the electron beam emitted from the electron
gun is usually controlled by an electric field lens, it has been also
known that when a magnetic field lens is superimposed upon the electric
field lens, it is possible to reduce the spherical aberration effectively.
As the magnetic field lens as described above, a magnetic field formed by
permanent magnets of an ion pump is usually used. The electron gun
provided with the magnetic field lens as described above is referred to as
a magnetic field immersion type electron gun. The ion pump of the magnetic
field immersion type electron gun is a vacuum pump for evacuating a
predetermined vessel of an apparatus (e.g., an electron microscope) to
which an electron gun is attached.
In the ion pump, electrons are emitted from a cathode thereof against
particles within the vacuum vessel for ionization, and the ionized charged
particles are trapped on an anode of the ion pump as getter to evacuate
the vessel. In the above-mentioned ion pump, the charged particles in the
vessel are moved as by a cyclotron based upon the magnetic field of the
permanent magnets, so that the frequency of collisions can be increased
effectively to obtain a high vacuum. As the ion pump as described above, a
coaxial ion pump disposed coaxially with the optical axis of the electron
gun is widely used.
In the case where the coaxial ion pump is used, a magnetic field lens is
formed by the utilization of the magnetic field generated by the permanent
magnets of the coaxial ion pump, and the formed magnetic field lens is
superimposed upon the cathode of the electron gun or the electric field
lens to construct the magnetic field immersion type electron gun. In this
case, it is possible to improve the aberration of the electron gun of
field emission type, in particular.
Accordingly, the prior art techniques related to the present invention are
both the magnetic field immersion type electron gun and the coaxial ion
pump.
In the field emission type electron gun, an electron beam emitted at a
large solid angle from an apex of emitter cathode must be converged by an
emitter cathode. The electron beam is converged conventionally by use of
an electric field lens. However, there exists such a drawback that the
diameter of the beam increases due to the spherical aberration of the
electric field lens and thereby the average intensity of the electron beam
tends to be lowered.
To overcome this problem, a method of obtaining a complex lens of both the
magnetic field lens and the electric field lens by superimposing a
magnetic field upon the electron gun lens formed by an electrostatic field
lens or by replacing a part of the electron gun lens with a magnetic field
lens has been so far proposed, which is referred to as a magnetic field
immersion type electron gun (e.g., M. Troyon: High current efficiency
field emission gun system incorporating a pre-accelerator lens. Its use in
CTEM. Optik, 57, 401 (1980)). In this document, Troyon has improved the
current density by about 6 times by replacing a first anode of a
three-electrode electron gun with a magnetic field lens generated by an
electromagnet. In this case, the magnetic field lens must be positioned
within the electron gun chamber. However, in order to operate the electric
field emission type electron gun stably, since a baking (at 200.degree. to
300.degree. C.) is required to evacuate the electron gun chamber less than
an ultra-high vacuum of 10.sup.-9 Torr, the magnetic field lens must be
proof against the baking temperature. However, it is practically difficult
to form the magnetic field lens so as to be proof against this high baking
temperature.
To solve this problem, a relatively practical structure of the magnetic
field immersion type electron gun has been proposed, in which an
electromagnet is disposed outside the electron gun chamber so that the
magnetic field can be applied in the vacuum vessel from the outside
thereof (J. R. A. Cleaver; Field emission electron gun systems
incorporating single-pole magnetic field lenses. Optik, 52,293 (1978/79).
FIG. 4 shows a prior art structure proposed by Cleaver, in which a single
pole magnetic field lens 5 is mounted on an electron gun chamber. The
electron gun is of three electrode structure composed of a cathode 1, a
extraction electrode (Wehnelt) 2, and an electron gun anode 3. A single
pole magnetic field lens 8 is mounted on the upper wall of the vacuum
vessel of the electron gun as a magnetic field lens. As another method of
applying a magnetic field effectively from the outside of the electron gun
chamber, A. Takaoka et al. have proposed a structure in which a magnetic
field is superimposed from the side of the electron gun chamber (A.
Takaoka et al.: Improvement of beam characteristics by superimposing a
magnetic field on a field emission gun. J. Electron Microsc. 38, No. 2,83
(1989)).
The above-mentioned proposed method are of the type in which the magnetic
field lens is formed by an electromagnet. In this method, however, since
the vacuum system and the lens system are both provided as different
systems from each other, the structure is not only complicated, but also
the requirements are contradictory to each other as explained in further
detail below, thus causing one of problems when the magnetic field
immersion type electron gun cannot be put to practical use.
For instance, to realize high performance electron optical characteristics
(mainly with respect to the aberration characteristics), it is preferable
that the magnetic field intensity of the magnetic field lens superimposed
upon the electric field lens is high, that is, the lens intensity is
large. The best way of increasing the magnetic field intensity of the
magnetic field lens is to place the electromagnet for forming the magnetic
field lens near the electron gun. In this case, however, it is necessary
to place the electromagnet for forming the magnetic field lens within the
high vacuum of the electron gun chamber.
However, this method causes a reduction of the degree of vacuum due to gas
emitted from the electromagnet. Or else, heat resistances of the various
elements for constituting the electromagnet must be taken into account
during the baking for evacuation, which are not preferable.
When a high intensity magnetic field is required to be applied from the
magnetic field lens disposed outside the vacuum chamber, it is necessary
to increase the exciting current and the number of windings (i.e.,
ampere-turns) of the electromagnet for forming the magnetic field lens,
with the result that the size of the magnetic field lens is inevitably
increased extremely.
The degree of vacuum required to operate the field emission type electron
gun stably is 10.sup.-9 Torr in the case of the thermal electric field
emission type and 10.sup.-10 to 10.sup.-11 Torr in the case of the cold
cathode electric field emission type. To obtain the ultra-high vacuum as
described above in a small sized structure, the use of a coaxial ion pump
has been proposed (M. Miyoshi and Okumura U.S. Pat. No. 4,890,029.
Electron beam apparatus including a plurality of ion pump blocks, Dec. 26,
1989, and M. Miyoshi and Okumura U.S. Pat. No. 5,021,702. Electron beam
apparatus including a plurality of ion pump blocks, Jun. 4, 1991). In the
coaxial ion pump as disclosed in these Patents, some permanent magnets for
constructing the ion pump are arranged coaxially with the optical axis of
the electron gun and the electric field lens in symmetry about the same
optical axis, and further the evacuating portions of the electron gun and
the ion pump are formed integral with each other.
FIGS. 5 and 6 are cross-sectional views showing the prior art coaxial ion
pump. FIG. 5 shows a first example of the prior art coaxial ion pump. In a
cylindrical outer casing 10 for constructing a vacuum vessel of the
coaxial ion pump, an electron gun 11 is disposed at the central axis
thereof, and further an evacuation operation portion (i.e., ion pump
portion) 12 is disposed coaxially with the optical axis of the electron
gun 11 so as to enclose the electron gun 11.
The electron gun 11 is provided with an electron gun body 13, a cathode 14
mounted on the lower tip portion of the electron gun body 13, and a hollow
cylindrical anode 15 formed with a hole through which an electron beam B
emitted from an apex portion (the lower end cathode 14) of the electron
gun body 13 is passed. The electron gun lens portion, that is, the
electric field lens is composed of the cathode 14 and the anode 15. The
electron beam B emitted from the cathode 14 at a wide angle is converged
by the electric field lens and then introduced into another apparatus
arranged below.
The ion pump portion 12 is provided with a cylindrical inside permanent
magnet 16 disposed coaxially with the central axis, an outside permanent
magnet 17 also disposed coaxially with the central axis, and an ion getter
portion 18 interposed between the inside permanent magnet 16 and the
outside permanent magnet 17 also coaxially therewith. The inside permanent
magnet 16 and the outside permanent magnet 17 generate a magnetic field in
a radial direction of the outer casing 10, and the intensity of the
magnetic field is about 1500 to 2000 gauss.
The ion getter portion 18 is composed of a cylindrical ion pump anode 19
disposed coaxially with the optical axis of the electron gun 11, and a
plurality of cylindrical ion pump cathodes (gettering electrodes) 20 and
26 formed of titanium so disposed as to sandwich the ion pump anode 19
between both inside and outside thereof.
In the coaxial ion pump, since the electron beam b is self-shielded by the
inside permanent magnet 16, it is possible to reduce the influence of the
permanent magnets 16 and 17 upon the electron beam b. However, the fact
that the electron beam b is self-shielded from the magnetic field of the
inside permanent magnet 16 is not appropriate from the structural point of
view when considering the object that the magnetic field must be
positively superimposed upon the electron beam b emitted from the magnetic
field immersion type electron gun 11.
FIG. 6 shows a second prior art example of the prior art coaxial ion pump.
In this structure,.a plurality of coaxial ion pump anodes 31 are arranged
around an electron gun 13, and a pair of annular ion pump cathodes
(gettering cathodes) 32 are disposed so as to sandwich the ion pump anode
13 between both upper and lower sides of the ion pump anode 31. Further,
outside a vacuum vessel 30, two cylindrical permanent magnets 33 and 34
are disposed on both the upper and lower sides of the ion pump cathodes
32, respectively, so that a magnetic field can be applied in parallel to
the arrangement direction of the ion anode 31.
In the coaxial ion pump of this structure, it is possible to construct the
magnet field immersion type electron gun by superimposing the magnetic
field generated by the permanent magnets 33 and 34 upon the electric field
lens of the electron gun from the structural point of view. The
theoretical analysis results of a prototype electron gun of this type are
explained in a paper (Y. Yemazaki, M. Miyoshi, T. Nagai and Okumura:
Development of the field emission electron gun integrated in the sputter
ion pump, J. Vac. Sci, Techno., B9(6), November/December 2967 (1991).
Further, FIGS. 7 to 9 show another prior art example of the magnetic field
immersion type electron gun of the structure in which the coaxial ion pump
and the field emission type (FE) electron gun are integrated with each
other. FIG. 7 is a cross-sectional view showing the magnetic field
immersion type electron gun of the structure, in which the coaxial ion
pump and the electric field emission type (FE) electron gun are integrated
with each other. FIG. 8 shows the distribution of the magnetic field
intensity along the central axis of the magnetic field immersion type
electron gun in correspondence to the shape of the magnetic field lens
thereof; and FIG. 9 shows the magnetic fields generated by the permanent
magnets 43 and 44.
The magnetic field immersion type electron gun shown in FIG. 7 is basically
the same in structure as with the case shown in FIG. 6. In FIG. 7, two
cylindrical permanent magnets 43 and 44 are disposed outside a vacuum
vessel 40 under atmospheric pressure. To prevent the magnetic field from
being leaked toward the outside and further, to form a closed magnetic
circuit as perfect as possible, a malleable iron yoke 45 formed with a
cylindrical hollow portion and formed into U-shape in cross section is
provided in such a way that two permanent magnets 43 and 44 are attached
to the upper inside surface and the lower inside surface of the yoke 45.
Within a vacuum vessel 40, a cylindrical ion pump anode 46 is disposed
between the two permanent magnets 43 and 44. These permanent magnets 43
and 44, the ion pump anode 46, and the yoke 45 are all arranged coaxially
with the central axis 49 of the electron gun body 47 and the electron gun
lens 48. At the end of the electron gun 47, cathode 42 of the electron gun
is attached to the electron gun 47.
In this structure, it is possible to obtain the magnetic field distribution
as shown in FIG. 8 along the central axis thereof. However, there exists a
problem in that the magnetic field intensity direction is reversed at
points A; C and B, respectively.
The reason thereof will be explained with reference to FIG. 9. Here, the
assumption is made that the two permanent magnets 43 and 44 are mounted
upper and lower sides in such a way that the magnetic poles are arranged
as S.fwdarw.N.fwdarw.S.fwdarw.N. Then, the magnetic field As directed from
the permanent magnet 43 to the permanent magnet 44 near the ion pump anode
46, and further leaks (deviates) largely toward the central axis 49, as
shown in FIG. 9. This leaked magnetic field forms the magnetic field at
the middle peak point B (the maximum intensity at the middle height
position along the central axis 49) in the magnetic field distribution
shown in FIG. 8. On the other hand, in the vicinity of the edge portions
43a and 44a of the respective permanent magnets 44 and 43, leaked magnetic
fields flow toward the yoke 45, respectively. The direction (upward) of
this magnetic field is opposite to that (downward) of the magnetic field
at the middle portion. Accordingly, there exists the opposite magnetic
field intensity distribution having sub-peak points A and C both above and
below the main peak B, as shown in FIG. 8.
In the structure as shown in FIG. 6 or 7 although the magnetic field
distribution near the central axis as described above can be modified to
some extent by design, it is impossible to basically eliminate the
distribution as shown in FIG. 8.
In this case, the magnetic field at the middle main peak B or the lower
sub-peak A is to be used as the magnetic field lens of the magnetic field
immersion type electron gun. Here, it is preferable to use the main middle
peak B when only the lens effect of the magnetic field lens is taken into
account, because the magnetic field intensity is large. However, since
this main peak B is located as the fairly inner side of the coaxial ion
pump (at a central height position of the ion pump anode 46 on principle),
it is rather difficult to layout the mechanical elements. Further, since
the sub-peak A under the main peak B is superimposed upon the central
(optical) axis 49 of the electron beam, two magnetic field lenses are
eventually superimposed, so that there exists such a problem in that the
analysis is complicated and thereby the design is difficult. Accordingly,
in practice, the magnetic field immersion type lens is constructed by
superimposing the lower sub-peak A upon the electron gun lens.
Further, in this method, since the magnetic energy of the two permanent
magnets 43 and 44 is separated into three peaks A, Band C, so that the
utilization efficiency of the magnetic energy is not high. This implies
that in order to form a stronger magnetic field lens, a permanent magnet
of unnecessarily high surface magnetic flux density (high cost) must be
used or else the thickness of the permanent magnet cylinder must be
increased to increase the surface magnetic flux density, thus causing
another problem in that the weight and the size of the apparatus are
inevitably increased.
Further, another prior art method of constructing an ion pump coaxially
with the optical axis of the electron gun and further utilizing the
magnetic field generated by a permanent magnet of the ion pump as the
magnetic field lens is proposed by Wiesner (J. C. Wiesner et al. U.S. Pat.
No. 4,397,611. Particle beam instrumentation ion pump, Aug. 9, 1983).
In this method, a pair of ring-shaped permanent magnets are arranged on
both upper and lower sides coaxially with the electron gun in such a way
as to be supported by a yoke disposed inside the permanent magnets (on the
ion pump anode side sandwiched between a pair of the permanent magnets). A
magnetic field is applied to a vacuum vessel through the yoke, and the ion
pump anode is disposed within the vacuum vessel for vacuum evacuation. In
this case, the magnetic field generated by a pair of upper and lower
ring-shaped permanent magnets forms the magnetic field lens. The structure
is similar to that shown in FIG. 7.
However, this prior art structure involves the following problems:
(1) Since the magnetic field is applied from the permanent magnets through
the yoke to the ion pump anode portion at which the strongest magnetic
field intensity is required (at which the actual evacuation is effected,
and further the evacuation speed is proportional to the square of the
magnetic field intensity), the effective magnetic field intensity is
attenuated markedly or the magnetic force lines form a magnetic circuit in
the yoke, with the result that the evacuation speed is lowered.
(2) Since the ring-shaped permanent magnets and the yoke are combined with
each other, in the same-reason as explained with reference to FIGS. 8 and
9, a plurality of magnetic field peaks are inevitably formed around the
central axis.
As explained above, the prior art technique related to the magnetic field
immersion type electron gun involves the following problems:
(1) In the case of the method of forming the magnetic field lens in the
magnetic field immersion type electron gun by the electromagnet of the
coaxial ion pump, since a large magnetic field intensity is preferably
required to improve the magnetic field lens, in order to increase the
magnetic field intensity of the superimposed magnetic field lens, it is
necessary to place the electromagnet of the coaxial ion pump near the
electron gun or else to use a large electromagnet for generating a fairly
strong magnetic field.
In this case, in order to place the electromagnet near the electron gun,
the electromagnet must be disposed within the vacuum vessel. As a result,
in order to operate the electric field emission type electron gun stably
within an ultra-high vacuum, there arise various problems such as gas
discharge, heat resistance during baking, etc.
Further, although a large electromagnet can be used when disposed outside
the vacuum vessel, since the exciting current and the ampere-turns are
both increase, the size of the magnetic field lens inevitably increases.
(2) The method of controllably superimposing the magnetic field of the
permanent magnet in the coaxial ion pump upon the electric field lens of
the electron gun, that is, the method of forming the magnetic field
immersion type electron gun is the best method in practical use. In the
prior art method so far proposed, however, since the unitization
efficiency of the magnetic field of the permanent magnet is relatively low
and further a plurality of magnetic field intensity peaks are generated,
it is difficult to design the magnetic field lens. In addition, since the
peak position of the maximum magnetic field intensity of the best
performance is generated deep inside the ion pump, an electron gun lens of
complicated structure must be arranged within a narrow space, so that the
mechanical layout is markedly limited.
SUMMARY OF THE INVENTION
Accordingly, it is the object of the present invention to provide a
magnetic field immersion type electron gun, which proposes an optimum
construction with respect to the shape of yoke and the arrangement of
permanent magnets in a coaxial ion pump. In the proposed optimum
construction, a magnetic field immersion type electron gun of high
performance can be realized by forming the magnetic field lens efficiently
with the use of the magnetic field generated by the permanent magnets for
constituting the coaxial ion pump and further by superimposing the formed
magnetic field lens upon the electron gun. Further, the magnetic field
immersion type electron gun according to the present invention can satisfy
the basic object such that the electron gun chamber, in particular in the
vicinity of the cathode tip of the electron gun (at which an ultra-high
vacuum is required) is evacuated efficiently.
To achieve the above-mentioned object, the present invention provides a
magnetic field immersion type electron gun for controlling an electron
beam by an electric field lens and a magnetic field lens in combination,
comprising: an electron gun body for emitting the electron beam; an
electric field lens system disposed under said electron gun body, for
forming the electric field lens to control the electron beam emitted by
said electron gun body; and a coaxial ion pump including: a cylindrical
anode and at least one cathode; a pair of cylindrical permanent magnets
disposed coaxially with an optical axis of said electron gun body in such
a way as to sandwich said cylindrical anode and cathode therebetween and
magnetized in a mutually opposing direction of said two permanent-magnets;
and a hollow cylindrical yoke disposed also coaxially with the optical
axis thereof in such a way as to enclose said two permanent magnets within
a hollow portion thereof, said hollow cylindrical yoke being formed with
an annular yoke gap in a radially inner circumferential surface thereof to
leak out a magnetic flux flowing through said hollow cylindrical yoke
toward the optical axis thereof so that a magnetic field lens can be
formed and further superimposed upon the electric field lens formed by
said electric field lens system.
In the above-mentioned magnetic field immersion type electron gun, a height
position of the yoke gap along the optical axis roughly matches a vertical
position of an electron gun cathode attached to a tip portion of said
electron gun body.
Further, a pair of said permanent magnets are disposed in such a way as to
sandwich the ion pump anode and cathode in a direction perpendicular to
the optical axis.
Further, a pair of said permanent magnets are disposed in such a way as to
sandwich the ion pump anode and cathode in a direction parallel to the
optical axis.
Further, the yoke gap is formed at a lower end portion of the radially
inner circumferential surface of said hollow cylindrical yoke so as to
face an electron gun cathode.
Further, a pair of said permanent magnets are disposed within a vacuum
vessel of said coaxial ion pump.
Further, said permanent magnets are formed of samarium cobalt.
In the magnetic field immersion type electron gun according to the present
invention, a pair of cylindrical permanent magnets are disposed coaxially
with the optical axis of the electron gun body in such a way as to
sandwich the cylindrical anode and cathodes of the coaxial ion pump, and
further the two permanent magnets are magnetized in the mutually opposing
direction of the two permanent magnets. Further, the hollow cylindrical
yoke is disposed also coaxially with the optical axis in such a way as to
enclose the two permanent magnets within the hollow portion thereof.
Therefore, the direction of the magnetic flux formed by the two permanent
magnets is parallel to the optical axis in the radially inner
circumferential surface (on the innermost side facing the optical axis) of
the yoke. Further, since the yoke is formed with an annular yoke gap in
the radially inner circumferential surface of the yoke so as to leak out a
magnetic flux flowing through the yoke toward the optical axis, a magnetic
field can be formed in symmetry with respect to the optical axis. Further,
this magnetic field has a single peak value in its intensity distribution.
Accordingly, since the single magnetic field lens can be effectively
superimposed upon the electron gun lens, it is possible to realize the
magnetic field immersion type electron gun of high performance and
simultaneously the coaxial ion pump of small size and of high evacuation
capability.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a cross-sectional view showing a first embodiment of the magnetic
field immersion type electron gun according to the present invention;
FIG. 2 is a graphical representation showing the magnetic field intensity
distribution along the optical axis of the magnetic field lens formed
between a yoke gap shown in FIG. 1;
FIG. 3 is a cross-sectional view showing a second embodiment of the magnet
field immersion type electron gun according to the present invention;
FIG. 4 is a cross sectional view showing a prior art example of the
magnetic field immersion type electron gun having an electromagnet
disposed outside the electron gun chamber;
FIG. 5 is a cross sectional view showing a first prior art example of the
magnetic field immersion type electron gun which uses a coaxial ion pump;
FIG. 6 is a cross sectional view showing a second prior art example of the
magnetic field immersion type electron gun which uses a coaxial ion pump;
FIG. 7 is a cross sectional view showing a prior example of the magnetic
field immersion type electron gun in which a coaxial ion pump and an
electric field emission type electron gun are formed integral with each
other;
FIG. 8 is a graphical representation showing the magnetic field intensity
distribution along the central axis of the magnetic field immersion type
electron gun shown in FIG. 7; and
FIG. 9 is an illustration for assistance in explaining the magnetic force
lines generated by the magnetic field immersion type electron gun in
correspondence to FIGS. 7 and 8.
DETAILED DESCRIPTION OF THE EMBODIMENTS
A first embodiment of the magnetic field immersion type electron gun
according to the present invention will be described hereinbelow with
reference to FIGS. 1 and 2.
In FIG. 1, in a cylindrical outer casing 50 for constructing a vacuum
vessel of the coaxial ion pump, an electron gun 51 is disposed at the
central axis of the outer casing 50, and further an evacuation operation
portion (i.e., ion pump portion) 53 is also arranged coaxially with the
optical axis 52 of the electron gun 51 so as to enclose the electron gun
51.
The electron gun 51 is provided with an electron gun body 54, an electron
gun cathode 55 mounted on the lower tip portion of the electron gun body
54, and an electron gun lens system 56 disposed under the electron gun
body 54. The electron gun lens system 56 forms a hollow cylindrical
electric field lens, and has a hole through which an electron beam can be
passed at an upper end portion of the lens system 56. The electron gun
lens system 56 is an electric field lens portion of the electron gun lens,
which is composed of a extraction electrode 56a, a lens electrode 56b and
an anode electrode 56c.
An ion pump (evacuation) portion 53 is provided with a cylindrical inside
permanent magnet 57 disposed coaxially with the optical axis 52 of the
electron gun 51, a cylindrical outside permanent magnet 58 disposed
outside the inside permanent magnet 57 also coaxially with the optical
axis 52, a plurality of ion getter portions 59 interposed between the
inside permanent magnet 57 and the outside permanent magnet 58 in the
radial direction coaxially with these permanent magnets 57 and 58, and a
hollow cylindrical yoke 60 disposed coaxially with the optical axis 52 so
as to enclose the two permanent magnets 57 and 58 within its hollow
portion. These permanent magnets 57 and 58 are held by the yoke 60 formed
of an malleable iron.
The permanent magnets 57 and 58 are of SmCo (samarium cobalt) based
rare-earth element magnet. The SmCo based magnet is high in thermal
stability and excellent in irreversible temperature change characteristics
(the demagnetization ratio is as low as 1% or less at 150.degree. C. for
100 hours), which is sufficiently proof against backing temperature for
obtaining a high vacuum. Further, when being plated with nickel for
prevention of degasification, the SmCo based magnet can be sufficiently
proof within a vacuum as high as 10.sup.-10 Torr or less.
The inside permanent magnet 57 and the outside permanent magnet 58 are
magnetized in the radial direction perpendicular to the optical axis 52,
so that magnet flux is generated so as to pass through an ion pump anode
61 in the radial direction beginning from the optical axis 52 in a plane
perpendicular to the optical axis 52.
Further, as far as a radial magnetic field can be generated, without being
limited only to the cylindrical shape, any shape of a pair of the
cylindrical permanent magnets 57 and 58 can be adopted (e.g., polygonal
(e.g., octagonal) shape, etc.).
The ion getter portion 59 is composed of a cylindrical ion pump anode 61
disposed coaxially with the optical axis 52 of the electron gun 51, and
ion pump cathodes (a plurality of cylindrical gettering electrodes) 62
formed of titanium so disposed as to sandwich the ion pump anode 61
between both inside and outside thereof.
Further, an annular yoke gap 63 is formed coaxially with the optical axis
52 under the radially inner circumferential surface of the yoke 60 (the
position at which the yoke 60 faces the optical axis 52). This yoke gap 63
is so formed that the middle height position of the yoke gap 63 along the
optical axis 52 roughly matches the vertical position of the cathode 55
attached to the lower tip portion of the electron gun body 54.
This yoke gap 63 serves to leak out the magnetic flux flowing through the
yoke 60 toward the optical axis 52; that is, the magnetic flux is leaked
from the yoke (magnetic flux leakage) so that a magnetic field lens can be
formed. Further, since the electron gun portion 56 can be evacuated
through the yoke gap 63, the evacuation conductance can be improved, so
that the actual evacuation speed can be eventually increased.
The respective ion pump composing elements including the yoke 60, the two
permanent magnets 57 and 58, the ion pump anode 61 are all disposed within
the vacuum vessel 50 connected to two upper and lower outer flange
portions 64 and 65. The upper flange 64 is connected to a stage (not
shown) for supporting the electron gun 51, and the lower flange 65 is
connected to a column, respectively. Here, although the yoke 60 is
disposed within the vacuum vessel 50, it is of course possible to use the
yoke 60 in common as the outer casing of the vacuum vessel 50.
The operation of the first embodiment will be described hereinbelow.
The assumption is made that the inside permanent magnet 57 is magnetized to
S pole on the inner side and N pole on the outer side, and the outside
permanent magnet 58 is magnetized to S pole on the inner side and N pole
on the outer side. Then, as shown by arrows in FIG. 1, a magnetic flux can
be formed from the inside permanent magnet 57 to the outside permanent
magnet 58 in a plane perpendicular to the optical axis 52 so as to pass
through the ion pump anode 61 in the radial direction thereof. The
magnetic flux emitted radially from the outside permanent magnet 58 passes
through both the upper side and the lower side of the yoke 60 and then
returns to the inside permanent magnet 57. Since the yoke 60 is formed
with a single annular yoke gap 63, a magnetic flux leaks from the yoke 60
at the yoke gap 63 (magnetic flux leakage).
FIG. 2 shows a simulation result of the magnetic field intensity
distribution along the optical axis 52. In FIG. 2, the origin (zero
position) is determined at the middle height position of the yoke gap 63,
and the upper side position from this origin is determined to be positive
and the lower side position from this origin is determined to be negative.
The two permanent magnets 57 and 58 are formed of SmCo, and the surface
magnetic flux density thereof is 1500 gauss, respectively. Further, the
yoke 60 is formed of malleable iron. FIG. 2 indicates that a magnetic
field lens can be formed in such a way that a peak value of about 500
gauss is located at the middle height position of the yoke gap 63 (at the
cathode tip position in FIG. 1). In addition, this magnetic field lens has
only a single peak position.
The peak magnetic field intensity of 500 gauss is equivalent to an
objective lens operated at the several kilovolts in the acceleration
voltage range, which indicates that a strong magnetic field lens can be
formed. In addition, since the single peak magnetic field lens can be
formed and since the number of the formed lens is one, as already
explained under the Background of the Invention, it is possible to utilize
the magnetic energy of the permanent magnets efficiently and further to
facilitate the analysis of the magnetic field lens and thereby to optimize
the design of the whole electron gun.
As explained above, in the construction of the present embodiment, since
the yoke 60 is formed with an annular yoke gap 63 so that the magnetic
flux flowing through the radially inner circumferential surface (facing to
the optical axis 52) of the yoke 60 can be leaked out toward the optical
axis 52, it is possible to form the magnetic field having a single peak
value in symmetry with respect to the optical axis 52. As a result, the
magnetic field lens can be superimposed upon the electric field lens.
Further, since the magnetic field lens has a distribution of the magnetic
field intensity of single peak value, the magnetic energy of the permanent
magnets can be utilized effectively. Further, since only the single
magnetic lens is formed, the analysis of the magnetic field lens is easy
and thereby the whole electron gun can be easily designed under the
optimum conditions.
Further, since the electron gun lens portion 56 can be evacuated through
the yoke gap 63, the evacuation conductance can be improved and thereby
the actual evacuation speed can be increased.
Further, since the middle height position of the yoke gap 63 is roughly the
same as the vertical position of the cathode 55 attached to the lower tip
portion of the electron gun body 54, it is possible to match the magnetic
field lens formed by the magnetic field leaked from the yoke gap 63 with
the position of the cathode 55. As a result, since the electron beam
emitted from the cathode 55 can be immediately introduced into the
magnetic field lens, it is possible to control the electron beam into a
spherical wave under excellent conditions, as compared when the electron
beam is controlled by the magnetic field lens after having traveled at a
long distance, with the result that it is possible to reduce the spherical
aberration of the magnetic field lens effectively.
Further, since being formed of samarium cobalt, even if the permanent
magnets 57 and 58 are disposed within the vacuum vessel 50 of the ion
pump, the permanent magnets can be proof against the high baking
temperature. Further, when the permanent magnets are plated with nickel as
the countermeasure against degasification, it is possible to obtain a
vacuum as high as 10.sup.-10 Torr or less.
A second embodiment of the present invention will be described hereinbelow
with reference to FIG. 3.
In this embodiment, two permanent magnets 70 and 71 of the ion pump are
disposed coaxially with the optical axis 52 in such a way as to sandwich
the ion pump anode 61 in the axial direction. Further, the two ion pump
cathodes 73 and 74 are disposed coaxially with the optical axis 52 in such
a way as to sandwich the ion pump anode 61 also in the axial direction.
The yoke 60 is formed with a yoke gap 63. In the construction as described
above, it is possible to form a magnetic field lens by the magnetic flux
leaked from the yoke gap 63 at the position at which the cathode 55 of the
electron gun is located, so that the same effect as with the case of the
first embodiment can be also obtained.
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